Although commercial application of ultrasonic holography as been accurately pursued by many persons in the scientific and industrial communities for many years, only limited results have been obtained even though it was once thought that ultrasonic holography held great promise. It was felt that the application of ultrasonic holography was particularly applicable to the fields of nondestructive testing of materials and medical diagnostics of soft tissues that are relatively transparent to ultrasonic radiation. One of the principal problems that has been encountered and not effectively resolved is the difficulty of obtaining quality and consistent images.
Solutions to this problem have been elusive, in part because of the difficulty in identifying the many causes that contribute to the problem. It is believed that one of the major problems has been the difficulty in devising or constructing quality large field ultrasonic imaging lenses. It appears that prior large field ultrasonic lenses exhibit substantial wave distortions and aberrations when used in a typical ultrasonic holographic imaging system such as illustrated in FIG. 1.
FIG. 1 shows a typical "real time" ultrasonic holographic imaging system generally designated with the numeral 10. The system 10 is intended to ultrasonically inspect the interior of an object 12 such as the soft tissue of a human limb. The ultrasonic holographic imaging system 10 generally has a hologram generating subsystem 14 for generating an ultrasonic hologram. The system 10 also includes a hologram viewing subsystem (optical-subsystem) 16 for optically viewing the interior of the object 12 from a first order refraction from the formed ultrasonic hologram.
The subsystem 14 includes an object ultrasonic transducer 18 for generating plane waves through a coupling medium 20 contained in a deformable membrane 22. The deformable membrane 22 intimately contacts the object 12 on one side and a deformable membrane 24 contacts the object on the other side to provide ultrasonic coupling with minimum energy loss or wave distortion. The deformable membrane 24 forms part of the side wall of a container 28 that contains a liquid coupling medium 30.
One of the principal components and the main subject of this invention is the provision of an ultrasonic imaging lens system 32 for viewing a large field and focusing at a desired object focal plane 34. The ultrasonic imaging lens system 32 focuses the ultrasonic energy onto a hologram detector surface 36. The ultrasonic imaging lens system 32 includes a large diameter object lens 38 that is moveable with respect to a large diameter lens 40 for adjusting the desired focal plane 34 in the object 12. The ultrasonic imaging lens system 32 includes a mirror 41 for reflecting the ultrasonic energy approximately 90.degree. and onto the hologram detection surface 36 to form the hologram.
A ultrasonic reference transducer 42 directs coherent ultrasonic plane waves through the liquid medium 30 at an off-axis angle to the hologram detector surface 36 to form the hologram. Preferably, the hologram detection surface 36 is the liquid/gas interface surface that is supported in an isolated dish or mini-tank 44.
The hologram viewing subsystem 16 includes an optical lens 45 to achieve an effective point source of a coherent light beam from a laser (not shown). The focused coherent light is reflect from a mirror 46 through a collimating optical lens 47 and then onto the hologram detector surface 36 to illuminate the hologram and generate diffracted optical images. The diffracted coherent light radiation containing holographic information is directed back through the collimating lens 47 and separated into precisely defined diffracted orders in the focal plane of the collimating lens 47. A filter 48 is used to block all but a first diffraction order from an ocular viewing lens 49 to enable a human eye, a photographic film or a video camera to record in "real time" the object at the object focal plane. As previously mentioned, although such a system is operable, it has been difficult to obtain quality and consistent images.
Two prior art large field ultrasonic lenses are described in U.S. Pat. No. 3,802,533 entitled "Improvements In and Relating To Ultrasonic Lenses" granted to Byron B. Brenden. More specifically, FIG. 2 of this application shows an ultrasonic liquid lens generally designated with the numeral 50 having flexible membrane films 52 surrounding a liquid lens 54. The liquid lens 54 includes convex liquid lens surfaces 56 and 58 forming a double convex liquid lens. Each of the flexible membrane films 52 is preferably formed of a stretched polymeric film in which each of the films has a thickness of less than one-quarter of the wave length of the ultrasonic wave length emitted from the transducer. The liquid lens preferably contains a liquid that is composed of trichloro-trifluoro-ethane (Freon 113). Other useful liquid lens materials included carbon tetrachloride, chloroform, ethyl bromide, ethyl iodide, methyl bromide, and methyl iodide.
A second prior art liquid lens is illustrated in FIG. 3 and identified with the numeral 62. The lens 62 has exterior membrane films 64 and interior membrane films 66. The interior membrane films 66 forms a main liquid lens 68 in an inner chamber. The main liquid lens 68 includes convex liquid lens surfaces 70 and 72 forming a double convex lens. The exterior membrane films 64 forms outer chambers that are filled with liquid lens material forming a convex outer liquid surface 74 and an inner concave liquid surface 76. It is indicated that the main liquid lens contains substantially the same liquid material as lens 54. It is indicated that the outer lens elements having surfaces 74 and 76 would be either water or a denser liquid having a different transmission velocity than water.
It is stated in U.S. Pat. No. 3,802,533 that one of the advantages of ultrasonic liquid lenses over solid ultrasonic lenses is the ability for imaging the ultrasonic wave front of one plane onto another plane through the liquid medium without significant energy loss or aberrations. Although such lenses may have been an improvement over what had previously been devised, it has been recognized that such lenses are not entirely satisfactory, and are difficult to provide with constant focal lengths during extended use. Additionally, such lenses were relatively difficult to manufacture and maintain. Furthermore, such lenses appear to have significant spherical aberrations.
FIG. 4 illustrates a general prior art lens 80 having spherical surfaces for directing outer rays 82 and inner rays 84 converging to a central axis. The outer rays 82 converge at a first focal plane 86, whereas the inner rays 84 converge at a focal plane 88. In an ideal lens, the rays 82 and 84 would converge at the same focal plane. The distance "A" between the focal planes 86 and 88 indicates the degree of spherical aberrations in the lens 80.
One of the principal objects and advantages of this invention is provide an improved solid ultrasonic imaging lens that overcomes many of the disadvantages of the previous lens systems to provide images of high quality.
These and other objects and advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.